Page 2 :
• Metameric segmentation or metamerism is an architectural body plan, in some animals in which the similar body segments and organ, systems are serially repeated one after another., • The similar body segments are called metameres or somites., • The animals shows such features called metamerically segmented., • Structurally each meta-mere or somite is constructed on the basis of, some fundamental plan and usually possesses a part of almost all the, body systems.
Page 3 :
Occurrence of Metamerism:, • Metamerism is first seen in annelids in animal kingdom., • Each segment usually contains appendages, muscles, nerves, blood, vessels, excretory organs and a pair of coelomic sacs which are, repeated in almost all segments., • It is also seen in kinorhynchs, arthropods and most chordates.
Page 4 :
• Metamerism first observed in Annelida in the animal kingdom., • The most successful animals of animal kingdom like arthropoda and, chordate will also show metameric segmentation., • In annelids the metameric segmentation is both external and, internal. The body is divided into a number of segments which, contain all body organs repeatedly but the alimentary canal is long, and straight tube extending through all the segments., • In arthropods the segmentation is external., • In chordates the segmentation is internal.
Page 5 :
• Characteristic Features of Metamerism:, • 1. Metamerism is always Very small to the intermediate (trunk), segments except the anterior acron (head) and a poste rior pygidium, or telson., • 2. Each metamere represents a mirror image of the other., • 3. Segmental structures are interdepend ent on each other., • 4. They are integrated into a single functional unit., • 5. All the segments of body work in coordination.
Page 7 :
Types of Metamerism:, • The metamerism in different groups is divided into the following, types:, • 1. True Metamerism:, • The true metamerism is one in which the segmentation of the body, develops by the segmentation of the mesoderm. It occurs in annelids,, arthropods and in most chordates., • The body of annelids consists of a number of segments and the, number remains constant in a particular species except in certain, cases of asexual reproduction., • New segments are not added to the body after maturation.
Page 8 :
• That means after the embryonic stage all the segments become the, same age., • Moreover, the segmental structures are inter dependent and, integrated so that the individuality of the body is preserved. New, segments arise at the posterior end in front of the pygidium., • Hence, newer segments occur at the posterior end and older, segments remain just behind the head.
Page 9 :
• 2.Homonomous Metamerism:, • If the segments or somites of the animal are all alike, the’, segmentation is called homonomous metamerism. It is seen in, annelids. Each metamere contains segmental blood -vessels, nerves,, nephridia and coelomoducts., • This is a primitive type of segmentation and is not found in any exist, ing animal because a few anterior segments are specialised to form, the head which is called cephalization., • A well-defined and well-organised head is lacking in annelids., • However, formation of a ‘head’ is suggested in polychaetes by, anteriorly placed struc tures and their association with parapodial, cirri.
Page 10 :
• 3.Heteronomous Metamerism:, • In arthropods and chordates the segments of the body are dissimilar, in different body regions and restricted only to certain organs. This, type of metamerism is called heteronomous metamerism.
Page 11 :
• 4.External Metamerism:, • In arthropods, the metamerism is exter nal. Internally the segments are not, marked by partitions., • 5. Internal Metamerism:, • In vertebrates there is internal metamer ism, seen in the embrayos and, confined to the muscular,, • skeletal (vertebrae and ribs) and nervous system., • 6. External and internal Metamerism:, • In Annelids, the metamerism is clearly visible both externally and, internally., • Externally it is marked by the constriction on the skin of the body and, internally it is marked by the partitions (septa).
Page 12 :
• 7.Complete Metamerism:, • When the segmentation is seen practically in all systems, the, metamerism is called complete, • metamerism. It is seen in annelids., • 8. Incomplete Metamerism:, • When the segmentation is not seen in all the organs, the metamerism, is called incomplete metamerism. It is seen in arthropods and, chordates.
Page 13 :
• Pseudo-metamerism or Strobilization:, • In contrast to true metamerism, pseudo-metamerism or strobilization, is seen in tapeworms (Platyhelminthes) where segmentation of the, body takes place by the segmentation of the ectoderm., • The body consists of a number of segments or proglottids which, varies in different individuals of the same species., • New segments are added to the body throughout life.
Page 14 :
• Significance of Metamerism:, • 1. It helps in locomotion, not only in burrowing but in all other types, of locomotion., • 2. Metamerism offers division of labour.
